The speed of sound in dry air at sea level at a temperature of 60 degrees Fahrenheit is just over 1,100 feet per second. When a jet fighter exceeds that velocity, shock waves that ripple through the atmosphere when air molecules are suddenly displaced by the plane create a loud noise called a sonic boom. The pilot does not hear the boom because he is traveling so fast it cannot catch up with him. I know this to be true firsthand because I used to make frequent business trips to various European countries and the Middle East, and when I was in a hurry, I would board the Concorde, a supersonic airliner jointly created by France and Great Britain. It flew on the edge of outer space at an altitude of 60,000 feet, with a cruising speed of close to 2,000 fps, about as fast as my Marlin Model 336 in 35 Remington launches a 200-grain bullet.
A bullet from a rifle that exceeds the speed of sound also creates a sonic boom, but it is much quieter because the mass of the bullet is tiny compared to that of a jet fighter. Attach an efficient suppressor to the rifle, and the much louder muzzle blast created by the exit of pressurized propellant gas into the atmosphere is reduced to a level that allows occasional shots to be comfortably fired without wearing hearing protection. Slow down the bullet to a velocity below the speed of sound, and the bullet striking steel or the bolt in an AR-15 cycling will be louder.
The Subsonic Movement
The subsonic movement as we know it today began in 1992 when J.D. Jones of SSK Industries created the 300 Whisper cartridge by necking down the 221 Remington Fireball case and loading it with 220-grain match-grade bullets. Intended for use in AR-15 rifles, the conversion required little more than switching barrels, and it used the same magazine as the 223 Remington. The cartridge picked up more momentum around 2010, when Advanced Armament Corporation renamed it 300 AAC Blackout and teamed up with Remington, who registered it with S.A.A.M.I.
Due to the rapidly growing popularity of attaching suppressors to the barrels of rifles, the 300 Blackout has become extremely popular outside of military circles, and Hodgdon has developed plenty of subsonic and supersonic load data for it and other cartridges, such as the 30-30 Winchester, 223 Rem., 308 Win., 350 Legend, 7.62x39mm Russian, 30-06, 458 SOCOM, and 45-70 Government. If what you are looking for is not in this edition of the Annual Manual, go online and check out Hodgdon’s Reloading Data Center. The gun club that I have been a member of for several decades is approaching 1,000 members, and I am amazed at how many are shooting suppressed rifles. Most are bolt actions, with the 308 Win. probably the most popular, although I also see autoloaders as well as Henry lever actions in 45-70, 30-30, and 44 Magnum.
Some of the smaller, high-expansion-ratio cartridges are ideally suited for subsonic handloads because the extremely small powder charges required for producing drastically reduced velocities occupy a large percentage of the powder cavities of their cases. Optimal examples in .30 caliber are the 300 Blackout, Bill Wilson’s 300 Hammer, the 7.62x39mm Russian, and the 30 Remington AR. The load density for most of the 300 Blackout subsonic loads developed by Hodgdon exceeds 90 percent. Due to its greater capacity, the 30-30 is not as efficient, but it is more so than the larger 308 Win., which is slightly more efficient than the 30-06. Even so, all can be successfully loaded to subsonic velocities, and all can be accurate.
Notes on Load Density
Load density indicates the amount of empty space inside a case from its web to the base of the bullet occupied by a powder charge. A charge that completely fills that space is considered to be a 100 percent density load, and velocity variation is likely to be low. If it fills only half the space, it is a 50 percent density load, and depending on the powder being loaded, velocity variation can be quite high. Due to the varying densities of various powders and variations in powder charge weights, load density in subsonic ammunition will vary considerably. Subsonic data with the highest load densities developed by Hodgdon that I received for several cartridges were 35 percent for the 223 Rem. (Accurate No. 2 powder), 38 percent for the 308 Win. (also Accurate No. 2), and 76 percent for IMR 4064 in the 45-70. Since Trail Boss is far more bulky than the other powders, load density for 13.5 grains behind the Rim Rock 405-grain cast bullet that I shot is 99 percent. Unfortunately, Australian Defense Industries (ADI) has not delivered any Trail Boss to Hodgdon for several years, and this is why staff members at Hodgdon’s ballistics lab have spent many hours developing subsonic loads with a variety of other powders. Increase expansion ratio beyond that of the 45-70 by switching to the 458 SOCOM, and for nine of the powders used by Hodgdon with the Hornady 410-grain SUB-X and ACME 576-grain bullets, load density slightly exceeds 100 percent due to slight powder charge compression. Those powders are Benchmark, IMR 3031, Accurate LT-30, Accurate LT-32, IMR 8208 XBR, Accurate 2495, IMR 4198, Accurate 2200, and Accurate 5744.
At the short distances slow-poke loads are usually shot, load density does not appear to be a determining factor in producing the best accuracy. Load density for 8.7 grains of Titegroup behind the Hornady 175-grain SUB-X in the 308 Win. is only 26 percent, yet when that load was shot in my Jarrett custom rifle, it averaged 0.46 inch at 100 yards. That rifle, by the way, has a Remington Model Seven action, McMillan stock, and a medium-heavy Shilen Select Match Grade barrel. It consistently shoots inside half MOA with a variety of full-power loads, and I have used it to take a lot of game. Due to the small charges of powders used in subsonic loads, a short section of wooden dowel should be used to check each case for an accidental double charge prior to bulletseating.
It should be noted that powder charges shown by Hodgdon under the “Maximum Loads” heading indicate the heaviest charge that could be used in the company’s pressure gun without exceeding subsonic velocities. It does not indicate a maximum-pressure charge. This becomes quite clear when comparing maximum pressures shown for full-power loads in the 300 Blackout and those listed for subsonic loads in that cartridge. Due to the absolute minimum chamber and bore dimensions of a pressure barrel, pressure and velocity of a particular powder charge/bullet weight combination can be higher than in many offthe-shelf, mass-produced rifles. A velocity ranging from 1,000 fps to 1,075 fps is the goal, and in order to reach that, the subsonic charge listed by Hodgdon may have to be increased slightly when loading for the typical factory rifle. On the other hand, some rifles may require a slight decrease in Hodgdon’s recommended charge weight.
Developing Loads
When developing a subsonic load with a new bullet, I begin with a recommended powder charge, and if velocity proves to be a bit low, the charge is gradually increased to 1,050 fps or so. Once that point is reached, 10 rounds are slow-fired over chronograph screens to confirm that each bullet received enough push to exit the barrel. Cerification can also be accomplished by shooting a paper target with a pause to peep through a spotting scope between each shot.
Drastically reducing a pressure-tested load should not be attempted, as doing so can leave a bullet stuck in the barrel. Should this happen, rest the rifle muzzle-up and pour a bit of Kroil penetrating oil down its bore and against the front of the lodged bullet. A wad of paper towel pushed into the chamber prevents the oil from entering the action. After a one-hour soaking, take a brass rod or hardwood dowel of a diameter matching bore diameter as closely as possible, insert it into the barrel from its breech end and against the base of the bullet, and push the bullet from the barrel. It may take several taps on the rod with a small plastic-head hammer to get things moving.
While the original intended role of rifle ammunition loaded to subsonic velocity was for use by military and law enforcement personnel against bad people, in some circles it is now being promoted for hunting big game. While some calibers deliver adequate incapacitation power for two-legged creatures, using most on animals larger than coyotes and possibly juvenile feral hogs makes no sense at all to me. Even when used on those, proper bullet placement is extremely important, and the distance has to be quite short. Anything less will result in wounded animals escaping to die a slow and painful death, and no true sportsman who believes in the ethical treatment of game animals wants any part of that. I occasionally hunt big game with suppressed rifles, but I always use full-power ammunition when doing so.
Before the introduction of SUB-X bullets by Hornady, a jacketed rifle bullet that would expand at subsonic impact velocities did not exist. Due to a soft lead core enclosed by a thin jacket with deep skiving at its hollow nose and the soft Flex-Tip insert, SUB-X bullets will expand at impact velocities as low as 900 fps. The .30-caliber 175-grain and .458-caliber, 410-grain versions are suitable for use in rifles with tubular magazines, although they can be used in other types of rifles as well. SUB-X construction meets FBI protocol for terminal ballistic test requirements.
When exiting the muzzle of a rifle at just under 1,100 fps, SUB-X bullets should expand out to about 200 yards, although residual energy at that distance would be only about 400 foot-pounds for the .30-caliber 190-grain bullet and 363 ft-lbs for the 175-grain bullet. That’s about the same as for the 25-20 Winchester cartridge at the muzzle of my Marlin 1894, and no experienced hunter would consider it powerful enough to use on anything much bigger than an unsuspecting groundhog, and then only at fairly close range. Shortening the distance increases energy delivery of the two SUB-X bullets but not by much since at 50 yards the figures are 483 ft-lbs and 437 ft-lbs. Moving to the bottom line, I see no logical reason to shoot deer or other big game with a subsonic loading of the 300 Blackout.
For hunting pigs and deer with a suppressed rifle, the 45-70 pushing the Hornady 410-grain SUB-X along at 1,050 fps is a far better option than cartridges of smaller calibers. In addition to punching a much bigger hole, the bullet strikes a 100-yard target with 1,000 ft-lbs of punch, about the same as for a supersonic loading of the 44 Remington Magnum from a Marlin 1894 or Henry Big Boy. The fairly soft Rim Rock 405-grain bullet I shot for this report should also work nicely on deer. Performance of the 410-grain SUB-X pushed to higher velocity should be even more effective on deer, and for the benefit of those who would like to give it a try, I have included a supersonic load with Accurate 2200 powder. The 458 SOCOM loaded with the 410-grain SUB-X and the 450 Bushmaster loaded with the 395-grain SUB-X would pretty much match 45-70 performance on deer.
Due to the noise reduction, I enjoy hunting with suppressed rifles, but I always use supersonic ammunition. I am reminded of a wonderful hunt for red stags and feral goats in Scotland where I used a Mauser rifle in 308 Win. and Hornady ammunition loaded with the 165-grain SST at just over 2,900 fps. Only two shots were fired during the entire hunt, so hearing protection was not needed. But when many shots will be fired at the range or during other activities, I always wear hearing protection when shooting a suppressed rifle.
Optimal Powders
Keep in mind that a subsonic load developed during cool weather can go supersonic when the ammunition is subjected to hot weather. That makes some members of Hodgdon’s Extreme family of reduced-temperature-sensitivity propellants, such as H4198, H322, and H4895, excellent choices for subsonic loadings of ammunition that will be subjected to wide temperature extremes.
With all that said, when choosing a powder for subsonic velocities from any cartridge, I prefer to start with one that, according to Hodgdon, produces the highest pressure because I want all the push I can get against the base of a bullet. As an example, of various powders used by Hodgdon when shooting 300 Blackout data with the Hornady 190-grain SUB-X, pressures ranged from 18,100 psi for H322 to 29,100 psi for IMR 4227, so between the two, I would go with IMR 4227. Top pressure producers with the Berger 230-grain Hybrid Target are 9.6 grains of IMR 4227 at 33,100 psi and 9.0 grains of W296 at 32,900 psi.
Other Considerations
Bullet rotation, and therefore its stability in flight, is determined by the rifling twist rate of a barrel and the level of velocity. A small increase or decrease in twist will have considerably more effect on bullet rotation than small increases or decreases in velocity. As an example, at an exit velocity of 1,050 fps, the finger-long Berger 230-grain Hybrid Target will stabilize from a rifle in 300 Blackout with a 1:7.5 or 1:8 twist, but it likely will not when exiting at the same velocity from a rifle in 308 Win. with a 1:10 twist, which is standard for that cartridge. In addition to being inaccurate, an unstable bullet could strike the baffles of a suppressor.
Because they are softer, thin-jacketed bullets are less resistant to being pushed through a barrel than bullets of monolithic construction, and for that reason they are less likely to stick in the barrel should velocity dip too far below the lower end of the subsonic velocity range. Examples would be a relatively soft bullet like the Hornady 150-grain A-Max and an extremely hard bullet like the Hornady 150-grain GMX from a rifle in 300 Blackout or 308 Win.
Due to the noise reduction, I believe having a suppressed rifle on hand for home protection is a good idea, and in that case subsonic ammunition is just the ticket. And for that I can think of nothing more suitable than a short lever-action rifle, such as the Marlin 1894 or Henry Big Boy in 44 Magnum/44 Special. Either carbine in 357 Magnum firing 38 Special handloads with a 158-grain cast or jacketed bullet at subsonic speed should also be an excellent option. A short bolt action, such as my Ruger American Ranch Rifle with a 16.12-inch barrel, in 300 Blackout or 308 Win. would not be a bad choice for repelling home invaders. A stock AR-15 will usually not function with subsonic ammo, although with certain modifications it can be made to do so.
Such a pity that suppressors for rifles were not as easy to find and to acquire while I was growing up as they are today. If they had been, I might still be able to hear the chirp of a cricket.